Electronic alignment of acousto-optic modulator for modulating a laser

ABSTRACT

A method and apparatus are provided to modulate a beam of laser light. An acousto-optic modulator (AOM) is positioned to receive continuous-wave (CW) laser light at an angle approximating the Bragg angle. Fine tuning of the modulation is then used to compensate for any variations in the actual angle from the Bragg angle. A photodiode provides a feedback signal that may be used to tune or adjust the operation of the AOM. The photodiode is positioned to receive laser light at the first order diffraction and provide a signal indicative of the magnitude of the first order diffraction laser light produced by the AOM. The photodiode signal is coupled to an RF frequency generator so that the RF frequency may be adjusted to vary the acoustic wave in the AOM to compensate for any variations in the Bragg angle.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to electronic displays, and, moreparticularly, to a multi-color Laser Projection Display (LPD).

2. Description of the Related Art

Single-color or monochrome LPDs have been implemented using araster-based scanning system. A raster-based LPD uses a laser andoscillating mirror(s) that move in horizontal and vertical directions toscan the laser light over a viewing screen in a raster pattern. Bycontrollably modulating the laser in time with the movements of themirror(s), a two-dimensional image can be produced. In fact, the LPD canproduce a high quality image, such as VGA or higher resolution bymodulating the mirrors at frequencies in the range of 110's and 100's ofMHz.

Monochrome displays, however, have limited utility, whereas full colordisplays are in wide use and are desired and accepted by the generalpublic. Full-color LPDs may be produced by controllably combining red,blue and green laser light to produce a wide spectrum of colors.Generally, red, blue and green lasers are commercially available, butnot in small-form factors, such as semiconductor laser diodes. In fact,while semiconductor laser diodes that emit in red and blue wavelengthsare available and can be directly modulated, there is no commerciallyavailable semiconductor laser that emits in the green wavelength. It isadvantageous to use a green laser since having red, green, and bluewavelengths enable a true full-color display. Also, human vision is moresensitive to green. Thus, for the same emitted laser power, a brighterdisplay is perceived if green laser light is included.

The present invention is directed to overcoming, or at least reducing,the effects of one or more of the problems set forth above.

SUMMARY OF THE INVENTION

In one aspect of the instant invention, a method is provided formodulating laser light. The method comprises delivering laser light toan optic modulator at an angle generally incident at the Bragg anglerelative to the optic modulator and adjusting the operation of the opticmodulator to compensate for at least a portion of a deviation of thelaser light from the Bragg angle.

In another aspect of the instant invention, a method is provided formodulating laser light. The method comprises delivering laser light toan optic modulator at an angle generally incident at the Bragg anglerelative to the optic modulator and adjusting the optic modulator toproduce a desired characteristic of laser light of at least one ordereddiffraction.

In yet another aspect of the instant invention, an apparatus is providedfor modulating laser light. The apparatus comprises a source of laserlight, an optic modulator, a sensor and a controller. The opticmodulator is arranged to receive laser light at an angle generallyincident at the Bragg angle relative to the optic modulator. The sensoris adapted to measure a characteristic of laser light of at least oneordered diffraction produced by the optic modulator. The controller isadapted to adjust the operation of the optic modulator to compensate forat least a portion of a deviation of the laser light from the Braggangle.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may be understood by reference to the followingdescription taken in conjunction with the accompanying drawings, inwhich like reference numerals identify like elements, and in which:

FIG. 1 is a stylistic block diagram of a top level view of oneembodiment of the present invention;

FIG. 2 is a stylistic view of a viewing surface shown in FIG. 1;

FIGS. 3A and 3B depict a top view of a scanning device at various timesduring its operation; and

FIG. 4 depicts one embodiment of an alignment set up using a photodiodeto measure the power in the diffracted laser beam, and using thephotodiode signal to tune the RF frequency.

While the invention is susceptible to various modifications andalternative forms, specific embodiments thereof have been shown by wayof example in the drawings and are herein described in detail. It shouldbe understood, however, that the description herein of specificembodiments is not intended to limit the invention to the particularforms disclosed, but on the contrary, the intention is to cover allmodifications, equivalents, and alternatives falling within the spiritand scope of the invention as defined by the appended claims.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

Illustrative embodiments of the invention are described below. In theinterest of clarity, not all features of an actual implementation aredescribed in this specification. It will of course be appreciated thatin the development of any such actual embodiment, numerousimplementation-specific decisions must be made to achieve thedevelopers' specific goals, such as compliance with system-related andbusiness-related constraints, which will vary from one implementation toanother. Moreover, it will be appreciated that such a development effortmight be complex and time-consuming, but would nevertheless be a routineundertaking for those of ordinary skill in the art having the benefit ofthis disclosure.

The following co-pending applications are hereby incorporated byreference herein in their entirety: Method and Apparatus for Aligning aPlurality of Lasers in an Electronic Display Device, by Mik Stern et.al.; Method and Apparatus for Controllably Reducing Power Delivered by aLaser Projection Display, by Mik Stern et. al.; Method and Apparatus forDisplaying Information in Automotive Applications Using a LaserProjection Display, by Narayan Nambudiri et. al.; Method and Apparatusfor Providing an Interface Between a Liquid Crystal Display Controllerand a Laser Projection Display, by Narayan Nambudiri et. al.; A ColorLaser Projection Display by Paul Dvorkis et. al.; Method and Apparatusfor Capturing Images Using A Color Laser Projection Display, by ChinhTan et. al.; A Laser Projection Display, by Ron Goldman et. al.; Methodand Apparatus for Controllably Compensating for Distortions in a LaserProjection Display, by Carl Wittenberg et. al.; and Method and Apparatusfor Controllably Modulating a Laser in a Laser Projection Display, byDmitriy Yavid et. al.

Turning now to the drawings, and specifically referring to FIG. 1, astylistic block diagram of a laser projection display (LPD) 100, inaccordance with one embodiment of the present invention, is shown. Inthe illustrated embodiment, the LPD 100 includes three lasers 102, 104,106, each capable of emitting a beam of light 108, 110, 112 consistingof a unique color, such as red, green or blue. Those skilled in the artwill appreciate that the number of lasers and the color of light emittedtherefrom may be varied without departing from the spirit and scope ofthe instant invention.

The lasers 102, 104, 106 are arranged in a common plane 114 with thebeams of light 108, 110, 112 being angularly directed relative to oneanother to fall on a substantially common location 116 on a firstscanning device, such as a first scanning mirror 118, from where theyare reflected as beams of light 120, 122, 124. In the illustratedembodiment, the first scanning mirror 118 oscillates on an axis 120 at arelatively high rate (e.g., about 20-30 KHz). Rotation or oscillation ofthe first scanning mirror 118 causes the beams of light 108, 110, 112 tobe moved. That is, as the angular position of the first scanning mirror118 alters, so to does the angle of reflection of the beams of light120, 122, 124 from the first scanning mirror 118. Thus, as the mirroroscillates the reflected beams of light 120, 122, 124 are scanned toproduce movement of the beams of light 120, 122, 124 along one componentof the two-dimensional display.

The second component of the two-dimensional display is produced by asecond scanning device, such as a mirror 126. In the illustratedembodiment, the second mirror 126 is coupled to a motor 128 at a pivotpoint 130 so as to produce rotational or oscillating movement about anaxis that is substantially orthogonal to the axis of rotation of thefirst mirror 118. The beams of light 120, 122, 124 are reflected off ofthe second mirror 126 as beams of light 132, 134, 136 and directed to aviewing surface 138. The viewing surface 138 may take on any of avariety of forms without departing from the spirit and scope of theinstant invention. For example, the viewing surface 138 may be a fixedscreen that may be front or back lit by the lasers 102, 104, 106 and maybe contained in a housing (not shown) that is common with the LPD 100,or alternatively, the viewing surface 138 may take the form of anyconvenient, generally flat surface, such as a wall or screen, spacedfrom the LPD 100.

The second mirror 126 oscillates or rotates at a relatively slow rate,as compared to the rate of the first mirror 118 (e.g., about 60 Hz).Thus, it will be appreciated that, as shown in FIG. 2, the beams oflight 132, 134, 136 generally follow a path 140 on the display surface138. Those skilled in the art will appreciate that the path 140 issimilar in shape and concept to a raster scan commonly employed incathode ray tube televisions and computer monitors.

While the instant invention is described herein in the context of anembodiment that employs separate first and second scanning mirrors 118,126, those skilled in the art will appreciate that a similar path 140may be produced by using a single mirror. The single mirror would becapable of being moved about two axis of rotation to provide the fastand slow oscillating movements along two orthogonal axes.

As is apparent from FIG. 1, owing to the angular positioning of thelasers 102, 104, 106, even though the lasers 102, 104, 106 have beenarranged mechanically and optically to deliver the beams of light 108,110, 112 within the same plane 114 and at the same point (on therotational axis 120) on the mirror 118), each has a different angle ofreflection, which causes the beams of light 120, 122, 124 to diverge. Acontroller 142 is provided to controllably energize the lasers 102, 104,106 to effectively cause the beams of light 120, 122, 124 to becollinear, such that they may be reflected off of the second mirror 126and delivered to the same point on the viewing surface 138 relativelyindependent of the distance of the viewing surface 138 from the secondmirror 126.

Turning now to FIGS. 3A and 3B, the operation of the controller 142 tocause the beams of light 120, 122, 124 to be collinear is discussed. Tosimplify the discussion, only two lasers 102, 104 are illustrated inFIG. 3, but those skilled in the art will appreciate that the conceptsdiscussed herein may be extended to three or more lasers withoutdeparting from the spirit and scope of the instant invention. As shownin FIG. 3A, if the lasers 102, 104 are energized simultaneously, thereflected beams of light 120, 122 diverge. However, as shown in FIG. 3B,if the lasers 102, 104 are energized at slightly different times, thenthe beams of light 120, 122 can be made to follow a single, common path(i.e., the beams of light 120, 122 are collinear). For example, if thelaser 102 is energized at a first time t1, then the mirror 118 will beat a first position, as represented by the solid lines, and the beam oflight 108 will reflect off of the mirror 118 as the beam of light 120.Subsequently, if the laser 104 is energized at a second time t2, thenthe mirror 118 will be at a second position, as represented by thedashed lines, and the beam of light 110 will reflect off of the mirror118 as the beam of light 122. By precisely controlling the time t2, themirror 118 will be in a position to accurately reflect the beam of light122 along substantially the same path as the beam of light 120.

Thus, through the operation of the controller 142, the beams of light120, 122 are substantially collinear, but are slightly displaced intime. That is, the beams of light 120, 122 will now both be projectedonto substantially the same point on the display surface 138, but atslightly different times. However, owing to the persistence of the humaneye, the variation in timing is not detectable. That is, in the case ofthe three laser system described in FIG. 1, each of the lasers 102, 104,106 will controllably deliver laser light of a unique color andintensity to substantially the same point on the viewing surface 132within a relatively short window of time. The human eye will not detectthe three separate colors, bur rather will perceive a blending of thethree light beams such that a consistent and desired hue appears at thatpoint on the viewing surface. Those skilled in the art will appreciatethat this process may be repeated numerous times along the path 140 torecreate a picture on the viewing surface 132.

As discussed above, semiconductor laser diodes that emit in red and bluewavelengths are commercially available and can be used directly as thelasers 102, 104 to produce beams of red and blue laser light in theraster based system described above. A small form factor laser capableof producing a green beam of laser light has heretofore been generallyunavailable. Continuous-wave (CW) green laser light has been created byfrequency-doubling an Nd:YAG or Nd:YVO4 infrared solid-state laser;however, the relaxation lifetime of such solid-state lasers may be tooslow for some laser display applications. That is, these frequencydoubled lasers may not be modulated sufficiently fast to produce a highresolution display. The instant invention, however, demonstrate that itis possible to modulate an infrared semiconductor laser diode to createdisplays of red, green or blue colors.

FIG. 4 illustrates a block diagram of one embodiment of the laser 106arranged to produce green laser light. Generally, an acousto-opticmodulator (AOM) 400 may be configured to modulate a CW laser beam, suchthat green laser light may be produced.

The laser beam may be arranged to be generally incident at the Braggangle to the AOM 400. The interaction of the acoustic waves in the AOM,400 generated by an RF frequency, and the laser beam creates diffractionorders. The diffracted laser beam is thus modulated. For example, whenthe AOM 400 is producing an acoustic signal and modulating the laserbeam, the various diffraction orders are produced. However, when the AOM400 is not modulating the laser beam (e.g., no acoustic signal ispresent), then the various diffraction orders are not present (exceptfor the 0^(th) order). That is, the various diffraction orders may beturned “off and on,” producing a modulated laser beam. Those skilled inthe art will appreciate that while any of various diffracation ordersmay be utilized, the first order diffraction typically contains the mostenergy, and thus, is normally the ordered diffraction that is utilizedby the system.

The Bragg relationship is captured by the following expression:θ_(Bragg) =λf _(c)/2V _(α)

-   -   where θ_(Bragg) is the Bragg angle and is typically on the order        of 1 degree, X is the laser wavelength, f_(c) is the RF        frequency that generates the acoustic wave, and V_(a) is the        speed of the acoustic wave in the AOM medium.

To ensure high diffraction efficiency, the AOM may be generally alignedto an accuracy of about {fraction (1/10)}^(th), of the Bragg angle or0.1 degrees, or less. A mechanical alignment system capable of thislevel of accuracy may be difficult to achieve and maintain in a consumerdisplay product. At a minimum, the mechanical alignment mechanism wouldadd size and cost to the product. Moreover, the quality of the alignmentmay vary over time or even with changes in temperature and/or humidity.In any event, such a mechanical system would likely prove to be ratherdelicate, limiting its application to laboratory type environments.

Instead of fine aligning the AOM 400 to the laser beam mechanically, itis possible to tune the required Bragg angle to match the existingalignment of the laser/AOM assembly by changing the RF frequency thatgenerates the acoustic wave within the AOM 400. That is, by fine tuningthe RF frequency applied to the AOM 400, the operation of the AOM 400may be adjusted to compensate for any misalignment between the AOM 400and the laser beam. The Bragg angle changes linearly to the RFfrequency, as is evident by the above expression.

FIG. 4 shows one embodiment of an alignment set up using a photodiode402 to measure the power in the diffracted laser beam. The photodiodesignal is coupled to an RF frequency generator 404, which is preset toproduce a generally desired RF frequency signal. The photodiode signaloperates to fine tune the RF frequency to compensate for any variationsin the Bragg angle. The RF frequency can be tuned by various well knownstructures, such as capacitance or inductance tuning, or a voltagecontrolled oscillator. The photodiode 402 may be positioned to receiveone or more of the ordered beams produced by the AOM 400. If thephotodiode 402 is positioned to capture the signal in the zeroth orderbeam (as shown), which is a modulated version of the incident laser beamalong its path of propagation, the control loop is configured to stopfine tuning the RF frequency generator 404 when minimum power ismeasured by the photodiode 402. If the photodiode is positioned tocapture the 1^(st) order diffracted beam, which is usually the beam usedfor display, the control loop stops adjusting the RF frequency generator404 when maximum power is measured by the photodiode 402.

Thus, the electronic tuning of the Bragg angle at best can reduce oreliminate fine mechanical alignment. At the minimum, it provides anadditional adjustment for the alignment procedure.

Unless specifically stated otherwise, or as is apparent from thediscussion, terms such as “processing” or “computing” or “calculating”or “determining” or “displaying” or the like, refer to the action andprocesses of a computer system, or similar electronic computing device,that manipulates and transforms data represented as physical, electronicquantities within the computer system's registers and memories intoother data similarly represented as physical quantities within thecomputer system's memories or registers or other such informationstorage, transmission or display devices.

Those skilled in the art will appreciate that the various system layers,routines, or modules illustrated in the various embodiments herein maybe executable control units. The control units may include amicroprocessor, a microcontroller, a digital signal processor, aprocessor card (including one or more microprocessors or controllers),or other control or computing devices. The storage devices referred toin this discussion may include one or more machine-readable storagemedia for storing data and instructions. The storage media may includedifferent forms of memory including semiconductor memory devices such asdynamic or static random access memories (DRAMs or SRAMs), erasable andprogrammable read-only memories (EPROMs), electrically erasable andprogrammable read-only memories (EEPROMs) and flash memories; magneticdisks such as fixed, floppy, removable disks; other magnetic mediaincluding tape; and optical media such as compact disks (CDs) or digitalvideo disks (DVDs). Instructions that make up the various softwarelayers, routines, or modules in the various systems may be stored inrespective storage devices. The instructions when executed by thecontrol units cause the corresponding system to perform programmed acts.

The particular embodiments disclosed above are illustrative only, as theinvention may be modified and practiced in different but equivalentmanners apparent to those skilled in the art having the benefit of theteachings herein. Furthermore, no limitations are intended to thedetails of construction or design herein shown, other than as describedin the claims below. Consequently, processing circuitry required toimplement and use the described system may be implemented in applicationspecific integrated circuits, software-driven processing circuitry,firmware, programmable logic devices, hardware, discrete components orarrangements of the above components as would be understood by one ofordinary skill in the art with the benefit of this disclosure. It istherefore evident that the particular embodiments disclosed above may bealtered or modified and all such variations are considered within thescope and spirit of the invention. Accordingly, the protection soughtherein is as set forth in the claims below.

1. A method for modulating laser light, comprising: delivering laserlight to an optic modulator at an angle generally incident at the Braggangle relative to the optic modulator; and adjusting the operation ofthe optic modulator to compensate for at least a portion of a deviationof the laser light from the Bragg angle.
 2. A method, as set forth inclaim 1, wherein the optic modulator is an acoustic optic modulatoradapted to produce an acoustic wave as a function of a radio frequencysignal, and wherein adjusting the operation of the optic modulator tocompensate for at least a portion of a deviation of the laser light fromthe Bragg angle further comprises measuring a characteristic of laserlight of at least one ordered diffraction produced by the opticmodulator, and adjusting the radio frequency signal as a function of themeasured characteristic.
 3. A method, as set forth in claim 2, whereinmeasuring a characteristic of laser light of at least one ordereddiffraction produced by the optic modulator, and adjusting the radiofrequency signal as a function of the measured characteristic furthercomprises measuring intensity of laser light of at least one ordereddiffraction produced by the optic modulator, and adjusting the radiofrequency signal as a function of the measured intensity.
 4. A method,as set forth in claim 3, wherein measuring intensity of laser light ofat least one ordered diffraction produced by the optic modulator, andadjusting the radio frequency signal as a function of the measuredintensity further comprises measuring intensity of laser light of atleast the first ordered diffraction produced by the optic modulator, andadjusting the radio frequency signal as a function of the measuredintensity.
 5. A method, as set forth in claim 3, wherein measuringintensity of laser light of at least the first ordered diffractionproduced by the optic modulator, and adjusting the radio frequencysignal as a function of the measured intensity further comprisesmeasuring intensity of laser light of at least the first ordereddiffraction produced by the optic modulator, and adjusting the radiofrequency signal to substantially maximize the measured intensity.
 6. Amethod, as set forth in claim 3, wherein measuring intensity of laserlight of at least one ordered diffraction produced by the opticmodulator, and adjusting the radio frequency signal as a function of themeasured intensity further comprises measuring intensity of laser lightof at least the zeroth ordered diffraction produced by the opticmodulator, and adjusting the radio frequency signal as a function of themeasured intensity.
 7. A method, as set forth in claim 6, whereinmeasuring intensity of laser light of at least the zeroth ordereddiffraction produced by the optic modulator, and adjusting the radiofrequency signal as a function of the measured intensity furthercomprises measuring intensity of laser light of at least the zerothordered diffraction produced by the optic modulator, and adjusting theradio frequency signal to substantially minimize the measured intensity.8. A method, as set forth in claim 1, wherein the optic modulator isadjustable to vary an intensity of at least one ordered diffractionproduced thereby, and wherein adjusting the operation of the opticmodulator to compensate for at least a portion of a deviation of thelaser light from the Bragg angle further comprises measuring theintensity of laser light of at least one ordered diffraction produced bythe optic modulator, and adjusting the optic modulator to produce adesired intensity of laser light of at least one ordered diffraction. 9.A method, as set forth in claim 4, wherein adjusting the optic modulatorto produce a desired intensity of laser light of at least one ordereddiffraction further comprises adjusting the optic modulator tosubstantially maximize the desired intensity of laser light of at leastone ordered diffraction.
 10. A method for modulating laser light,comprising: delivering laser light to an optic modulator at an anglegenerally incident at the Bragg angle relative to the optic modulator;and adjusting the optic modulator to produce a desired characteristic oflaser light of at least one ordered diffraction.
 11. A method, as setforth in claim 10, wherein the optic modulator is an acoustic opticmodulator adapted to produce an acoustic wave as a function of a radiofrequency signal, and wherein adjusting the optic modulator to produce adesired characteristic of laser light of at least one ordereddiffraction further comprises measuring a characteristic of laser lightof at least one ordered diffraction produced by the optic modulator, andadjusting the radio frequency signal as a function of the measuredcharacteristic.
 12. A method, as set forth in claim 11, whereinmeasuring a characteristic of laser light of at least one ordereddiffraction produced by the optic modulator, and adjusting the radiofrequency signal as a function of the measured characteristic furthercomprises measuring intensity of laser light of at least one ordereddiffraction produced by the optic modulator, and adjusting the radiofrequency signal as a function of the measured intensity.
 13. A method,as set forth in claim 12, wherein measuring intensity of laser light ofat least one ordered diffraction produced by the optic modulator, andadjusting the radio frequency signal as a function of the measuredintensity further comprises measuring intensity of laser light of atleast the first ordered diffraction produced by the optic modulator, andadjusting the radio frequency signal as a function of the measuredintensity.
 14. A method, as set forth in claim 13, wherein measuringintensity of laser light of at least the first ordered diffractionproduced by the optic modulator, and adjusting the radio frequencysignal as a function of the measured intensity further comprisesmeasuring intensity of laser light of at least the first ordereddiffraction produced by the optic modulator, and adjusting the radiofrequency signal to substantially maximize the measured intensity.
 15. Amethod, as set forth in claim 12, wherein measuring intensity of laserlight of at least one ordered diffraction produced by the opticmodulator, and adjusting the radio frequency signal as a function of themeasured intensity further comprises measuring intensity of laser lightof at least the zeroth ordered diffraction produced by the opticmodulator, and adjusting the radio frequency signal as a function of themeasured intensity.
 16. A method, as set forth in claim 15, whereinmeasuring intensity of laser light of at least the zeroth ordereddiffraction produced by the optic modulator, and adjusting the radiofrequency signal as a function of the measured intensity furthercomprises measuring intensity of laser light of at least the zerothordered diffraction produced by the optic modulator, and adjusting theradio frequency signal to substantially minimize the measured intensity.17. A method, as set forth in claim 13, wherein adjusting the opticmodulator to produce a desired intensity of laser light of at least oneordered diffraction further comprises adjusting the optic modulator tosubstantially maximize the desired intensity of laser light of at leastone ordered diffraction.
 18. An apparatus for modulating laser light,comprising: means for delivering laser light to an optic modulator at anangle generally incident at the Bragg angle relative to the opticmodulator; and means for adjusting the operation of the optic modulatorto compensate for at least a portion of a deviation of the laser lightfrom the Bragg angle.
 19. An apparatus for modulating laser light,comprising: a source of laser light; an optic modulator arranged toreceive laser light at an angle generally incident at the Bragg anglerelative to the optic modulator; and means for adjusting the operationof the optic modulator to compensate for at least a portion of adeviation of the laser light from the Bragg angle.
 20. An apparatus formodulating laser light, comprising: a source of laser light; an opticmodulator arranged to receive laser light at an angle generally incidentat the Bragg angle relative to the optic modulator; a sensor adapted tomeasure a characteristic of laser light of at least one ordereddiffraction produced by the optic modulator; a controller adapted toadjust the operation of the optic modulator to compensate for at least aportion of a deviation of the laser light from the Bragg angle.